Few news stories are as disturbing as young people who take up a gun and go on a rampage against schoolmates. Yet they exert a morbid fascination for many people because the phenomenon is horrifying and incomprehensible. Last week in southwestern Germany, a 17-year-old gunman shot up his former high school, killing 15 people before turning the gun on himself. But the problem is far more prevalent in the United States, where such names as Columbine High School killers Eric Harris and Dylan Klebold and Virginia Tech gunman Cho Seung-hui have acquired worldwide notoriety. Thankfully, such incidents never happen in Hong Kong. There is no doubt we too must have our share of mentally disturbed youngsters; it is only that we have better gun control. A new book by US child psychologist Peter Langman has made a serious attempt to explain such deadly behaviour. He believes it is rooted in severe mental disturbance such as psychosis and delusional paranoia, coupled with an existential rage that finds expression in mass murder. But he also argues that complex environmental, family and individual factors combine to serve as a trigger. This is all very well. Over the years, many sophisticated and inter-disciplinary theories have been advanced to explain the unexplainable. However, such mass murders could not have occurred without the means available for perpetuators to realise their dark rage. The heart of darkness and the nature of evil will always elude human understanding. But the varying means at the disposal of killers make all the difference in terms of the carnage and destruction that may result. A mad man wielding a knife can only slash and kill before he is tired or overcome; a deranged gunman is far more dangerous and harder to stop. In trying to advance plausible explanations, overseas social scientists and law enforcement officers may have overfocused on the details and neglected the obvious and bigger picture. In the US, gun ownership is constitutionally protected. So the most obvious solution - banning personal weapons - is out of the question. As a result, experts have resorted to complicated methods and preventive measures to predict and deter young killers. But they are effectively being asked to square the circle.